<!DOCTYPE html>
<style>
#target {
  width: 200px;
  height: 200px;
}
@keyframes expand {
  0% { --foo: 10; }
  100% { --foo: 200; }
}
.animate {
  background-image: paint(shdow);
  animation: expand 1s;
}
</style>
<div id="target"></div>;
<script id="code" type="text/worklet">
  registerPaint('shadow', class {
    static get inputProperties() { return ['--foo']; }
    paint(ctx, geom, properties) {
      let fooValue = parseFloat(properties.get('--foo').toString());
      ctx.fillStyle = 'green';
      ctx.fillRect(0, 0, fooValue, fooValue);
    }
  });
  </script>

  <script>
  CSS.registerProperty({
    name: '--foo',
    syntax: '<number>',
    initialValue: '10',
    inherits: false
  });
  </script>

  <script>
  var code = document.getElementById('code').textContent;
  var blob = new Blob([code], {type: 'text/javascript'});
  CSS.paintWorklet.addModule(URL.createObjectURL(blob)).then(function() {
    document.getElementById('target').classList.add('animate');
  });
